1. Field of the Invention
This invention relates to a vibration wave motor utilizing travelling vibration waves.
2. Description of the Related Art
The outline of the principle of a vibration wave motor utilizing travelling vibration waves is as follows.
Two groups of circumferentially arranged piezo-electric elements are secured to one surface of a ring-like vibration member of a resilient material whose full circumferential length is an integer times as great as a certain length .lambda. to thereby provide a stator. These piezo-electric elements in each group are arranged at a pitch of .lambda./2 so as to have opposite expansion-shrinkage characteristics alternately, and are disposed so that there is a deviation an odd number times as great as .lambda./4 between the two groups. The two groups of piezo-electric elements are provided with an electrode film. If an AC voltage is applied to only one of the two groups (hereinafter referred to as the A phase), a standing wave (wavelength .lambda.) of such flexural vibration that the central point, of each piezo-electric element and a point spaced apart by .lambda./2 therefrom are the positions of antinodes and the midpoint between the positions of the antinodes is the position of a node, is produced over the entire circumference of the vibration member. If an AC voltage is applied to only the other group (hereinafter referred to as the B phase), a standing wave is likewise produced, but the positions of the antinodes and node thereof deviate by .lambda./4 relative to the standing wave by the A phase. When AC voltages of the same frequency but having a time phase difference of 90.degree. therebetween are applied to the A and B phases at one time, the standing waves of the two are combined together with a result that a travelling wave (wavelength .lambda.) of flexural vibration vibrating in the circumferential direction is produced in the vibration member, and at this time, each polyhedral point of the vibration member having a thickness effects a kind of elliptical movement. Consequently, if for example, a ring-like moving member such as a rotor is kept in pressure contact with the surface of the vibration member, the moving member is subjected to circumferential friction from the vibration member and is rotatably driven thereby.
Also, a driving method is known wherein a piezo-electric element for vibration detection (hereinafter referred to as the S phase) is provided besides the groups of piezo-electric elements of the A and B phases. The piezo-electric element is driven, for example, at a frequency satisfying conditions for making a phase difference between the output from this S phase and the applied driving voltage to the groups of piezo-electric elements of A and B phases constant, thereby keeping the vibrating state of the vibration member constant and stabilizing the motor output.
FIG. 32 of the accompanying drawings diagrammatically shows the driving circuit of a conventional vibration wave motor, and FIG. 33 of the accompanying drawings shows the arrangement of A and B phase and S phase of a ring-like vibration member and the polarization pattern thereof.
The reference numeral 1 designates the vibration member of a vibration wave motor which is formed into a ring-like shape. An S phase piezo-electric element 1-1, an A phase piezo-electric element group 1-2 and a B phase piezo-electric element group 1-3 are secured to one surface of the vibration member 1 as by an adhesive agent, and a C phase electrode C which is an electrode common to these piezo-electric elements is likewise secured to the other surface of the vibration member 1.
The A phase piezo-electric element group 1-2 and the B phase piezo-electric element group 1-3 are adapted to be drive-controlled by a driving circuit comprising an oscillator 3, a 90.degree. phase shifter 4, amplifiers 5 and 6 and a vibration detection circuit 2 for detecting the vibrating state on the basis of the phase difference or amplitude information from the S phase The output of a piezo-electric element 1-1, and an AC signal from an oscillator 3 are directly input to one amplifier 5 and a time phase difference is input to the other amplifier 6 with a deviation of 90.degree. through a 90.degree. phase shifter 4 so as to drive the A phase piezo-electric element group 1-2 and the B phase piezo-electric element group 1-3, respectively, and at that time, the oscillator 3 is controlled by a vibration detection circuit 2 so that the motor is driven at a regular wave number while the phase or voltage amplitude of a travelling vibration wave formed on the vibration member 1 by the S phase piezo-electric element 1-1 is detected.
The frequency of the AC signal applied to the A phase piezo-electric element group 1-2 and the B phase piezo-electric element group 1-3 by the oscillator 3 is determined by the natural frequency of the vibration member 1. The wave number of the travelling vibration waves formed on the vibration member 1 is determined, for example, by the interval between adjacent piezo-electric elements differing in the direction of polarization in the A phase piezo-electric element group 1-2 provided on the vibration member 1 and the circumferential length of the vibration member 1, and in the case of FIG. 33, there are formed eight travelling waves.
In the above-described driving circuit of a conventional vibration wave motor, however, only a signal of a frequency for driving is applied to the A and B phase piezo-electric element groups of the vibration wave motor, and this has sometimes led to the production of audible sound.
That is, if the resonance frequency of the vibration wave motor is f.sub..gamma.(n) which is the resonance frequency in a vibration mode of wave number=n (the number of waves produced on the vibration member, which is a positive integer), when the resonance frequency f.sub..gamma.(n) is being applied to the vibration wave motor, other vibration mode, for example, vibration of a frequency f.sub..gamma.(n-1), may be excited depending on the, situation of the vibration wave motor (such as the fluctuation of the torque load of the motor, a variation in the applied pressure to the moving member and the vibration member, or the like), and at this time, vibration of a low frequency f.sub..gamma.(n) -f.sub..gamma.(n-1) may also be excited, and this is heard as audible sound.
Also, when the above-described self-excited vibration is produced, for example, by the frictional force between the vibration member and the moving member of the vibration wave motor and the vibration of a travelling vibration wave which is not as per the design value (differing also in wave number) is added, a signal voltage which is not as per the design value is also added to the signal output from the S phase and therefore, exact phase and voltage amplitude at a regular wave number are not obtained, and this has led to the undesirable possibility that the vibration wave motor cannot be normally drive-controlled.